Avoid use of <assert.h>, as the GNU Coding Standards hint that one

Avoid use of <assert.h>, as the GNU Coding Standards hint that one
should use `if (! x) abort ();' rather than `assert (x);', and anyway
it's one less thing to worry about configuring.  (hash_lookup,
hash_get_first, hash_get_next, hash_find_entry, hash_rehash,
hash_insert): Use abort rather than assert.